1. Field of the Invention
The present invention relates to a method of producing monolith catalysts for constituting catalysts to be used in a measure to purify the exhaust gases of vehicles.
2. Description of the Prior Art
Generally, in known catalysts of this kind, cordierite (i.e., a compound of Al.sub.2 O.sub.3, MgO, SiO.sub.2) is used as monolith catalyst carrier. However, it is small in surface area as the catalyst carrier and is short in gas contact so that sufficient activity is not obtained by itself. To provide the sufficient activity, alumina having the properties of high surface area is coated on the surface of the cordierite carrier, and the coated alumina layer is impregnated with noble metal such as platinum, palladium or the like to provide higher activity.
As a method of producing the alumina coat layer on the carrier surface, there was known a method of dipping the carrier in alumina slurry liquid provided through the blending, kneading of hydroalumina (to be added with the active alumina as occasional demands) in aqueous solution hydrochloride, and thereafter drying, burning it, for instance, as disclosed within Japanese Patent Publication No. 31,939/1982. A process of producing liquid composition for use in alumina coating was disclosed in this publication, comprising the steps of slowly adding 5 to 30 parts by weight of bochmite impalpable powder of 325 meshes or less, while 95 to 70 parts by weight of 2.5 to 0.1 normal inorganic-acid aqueous solution or 4.5 to 0.1 normal organic-acid aqueous solution was being violently agitated, thereafter further agitating them for 20 to 30 minutes, curing the bochmite grains at cold temperatures, under no-pressure so that the bochmite grains could not be visually recognized and might become 20 to 200 centi-poise (hereinafter referred to as cp) in viscosity. The alumina coat layer provided in this manner is comparatively good in adherence property with respect to the cordierite carrier in the normal endurance temperature region (for example, 850.degree. C. or less) so that the alumina coat layer can be sufficiently used. However, the alumina coat layer comes off in the temperature zone higher than 850.degree. C. by all means. Accordingly, in the use as the catalyst, the noble metal broke off and reduced due to the coming off of such alumina coat layer, thus resulting in reduced catalytic activity.
It seems that the coming-off phenomenon of the alumina coat layer is caused due to the use of the aqueous solution hydrochloride for the blending, kneading operation of the hydroalumina, to the uniform adhesion of the slurry liquid on the surface of the carrier caused due to the relatively inferior dispersibility of the alumina particles, to the unstable crystal structure of the alumina during the burning operation.